1. Field of the Invention
The present invention relates to a laser device that controls laser power. In particular, it relates to a laser device that stably controls very low laser power.
2. Description of the Related Art
Typically, the power of a high power laser device is as much as several kW. As publicly known, such high power laser is used for cutting thick plates, deep penetration welding and the like.
On the contrary, the laser device can output very low laser of 10% or less of nominal power. When a very low laser is output, it is possible to laser mark characters on a workpiece surface, drawing index lines used in the next stage and the like or modify the workpiece surface without melting the workpiece surface.
For example, when a laser cutting machine comprising a laser oscillator whose nominal power is 6 kW is configured for an average power of 4.5 kW at a pulse frequency of 700 Hz using oxygen assist gas, it can cut a steel plate of 30 mm thickness with feed speed of 550 mm/min. In addition, when such laser cutting machine is configured for an power of 50 W±10 W at a pulse frequency of 1000 Hz using the oxygen assist gas, laser marking can be performed on a steel plate surface with feed speed of 5000 mm/min.
However, when a very low laser of 10% or less of the nominal power is output, the laser device is sensitive to an external environment and temperature inside it and therefore the laser power tends to become unstable. This is because, in a region in which a very low laser is output, laser oscillation vary sensitively in response to injection of excitation energy.
In the above-mentioned laser oscillator whose nominal power is 6 kW, the energy injected into a laser medium to obtain the nominal power corresponds to 30 kW. Consequently, thermal conditions of the laser resonator including the laser medium and optics differ between before and after the nominal power is reached. In such circumstances, the very low laser power of 10% or less of the nominal power is severely affected by the thermal conditions of the laser oscillator and, as a result, the laser power becomes unstable.
In this regard, in Japanese Unexamined Patent Publication No. 2006-344722, in each of a plurality of laser excitation regions, discharge phase, duty cycle and pulse frequency are controlled differently from each other so that the very low laser power can be stably controlled.
However, the configuration disclosed in Japanese Unexamined Patent Publication No. 2006-344722 into consideration the plurality of laser excitation regions is essentially the same as the configuration in which a plurality of portions in one excitation region are selectively excited. Consequently, a ratio of variation of the laser power to the injection energy for the laser excitation is not changed. Thus, because the very low laser power sensitively responds to the variation of command values, also in Japanese Unexamined Patent Publication No. 2006-344722, there still remains a problem that it is difficult to control the very low laser power.
The present invention has been made in light of these circumstances and an object of the present invention is to provide a laser device with improved controllability that can stably output very low laser even though it is a high power laser.